1. Field of the Invention
The invention relates to panels utilized in construction of buildings for the purpose of absorbing electromagnetic waves, particularly in the frequency ranges of radio transmissions, television transmissions, and microwaves, and more particularly to such panels made up of two or more distinct materials, such as composites and multi-layered panels.
2. Statement of the Problem
For many years it has been recognized that reflection of electromagnetic waves from buildings and other structures causes problems, such as ghosts in television reception and static and noise in radio reception. This is a particularly significant problem in densely populated high technology societies, such as the large cities of the United States, Europe and Japan. In Japan, for example, in large cities a broadcast television electromagnetic impact statement is required before a large building may be constructed, and construction codes may require that buildings be constructed to avoid reflections of electromagnetic waves in the frequency range of radio, television and some microwaves, i.e. between 80 to 2400 megahertz. Transmission of electromagnetic waves through many building materials also has in some situations created problems of secrecy. For these reasons, extensive research has been performed to find building materials that will absorb electromagnetic radiation. See, for example, Investigation on Oblique Incident Charactenistics of Ferrite Absorbing Panels For TV Ghost Suppression, Hironobu Ito et al. Japan Broadcasting Corporation et al. (about 1994). Wave absorption panels for use in building construction generally comprise a support layer of concrete or other basic building material, a reflective layer that is usually a metal mesh or other conductive material, an absorbing layer that typically is a ferrite, and an external layer, such as a silicate building tile, to protect the absorbing layer from environmental effects. Other materials that have been used as an absorbing layer include conducting materials, such as carbon fibers, in a resin.
Since nearly all matter has a characteristic frequency at which it absorbs radiation, it is relatively easy to find a material that will absorb electromagnetic radiation over some narrow frequency ranges. For example, ferrites typically have an absorption peak roughly between 200 megahertz to 400 megahertz. It is much more difficult, if not impossible, to find a material that will absorb over a broad frequency range of several thousand megahertz, or even just a few hundred megahertz. Thus, multilayered structures comprising combinations of ferrites, conducting fibers in a resin, and other similar structures have been tried as wave absorbers.
It is known to use a quarter-wave plate to provide an electromagnetic wave absorber. In such an absorber, a thickness of material equal to one-quarter of a wavelength is placed in front of a 100% reflector, such as a metal layer. This absorption principal has not, up to now, been applied in attempting to make absorption panels for buildings because waves in the television frequency range are many meters long. Thus, such an absorber that is a few meters long would be excessively thick for use in a building.
The most successful materials for wave absorption panels, ferrites, are relatively heavy, must be up to a centimeter thick to be effective, and are relatively soft and therefore require an additional layer of building material, such as tiles, to protect them from the environmental effects. Thus, wave absorption panels known in the art are bulky and heavy, making the structure expensive and unwieldy to employ on an entire building, are not capable of absorbing over the wide frequency range necessary to include all electromagnetic waves commonly present in a large metropolitan area, or both. Moreover, the frequency at which conventional ferrites absorb is in the 200-400 megahertz range, while VHF television frequencies range from about 100 to 250 megahertz and UHF television frequencies range from about 450 megahertz up to about 800 megahertz. Therefore, it would be highly desirable to have a wave absorption panel that is relatively light and thin while at the same time absorbs over a wide frequency range including up to about 800 megahertz.
The prior art wave absorption panels generally are useful only in the frequency range of television electromagnetic waves, which are the waves in which the problems due to reflection are most widespread. However, problems with reflection of waves can have serious consequences in other specialized areas, such as radio LAN systems, which can lose data because of reflections, and airport radio control systems, in which clarity of signal can be a matter of life and death. It would be very desirable to have absorption panels that absorb strongly in the frequency ranges of these specialized uses.
It has also been found that, in practice, due to the proximity to electromagnetic wave sources of a narrow frequency, many construction sites have a negative impact on the electromagnetic environment only in a narrow frequency range. This range cannot be predicted in advance of knowing the location of a building to be constructed. Therefore, it would be highly useful to have an absorber panel and process of fabrication of absorber panels that are easily tuned to a specific frequency.